This invention pertains to the manufacture of flat articles, such as printed circuit boards, and the like. In manufacturing printed circuit boards, various chemicals are used to produce the desired circuit. Before certain processing steps have been performed, it is necessary to clean the boards to remove excess chemicals (such as copper oxides) or othe substances. It is also necessary to remove dirt which may have accumulated on the circuit board during prior processing.
In the prior art, it has been known to scrub the printed circuit board with a rotating brush positioned above or below the plane of the board. The flat board may thus be scrubbed while traveling through the manufacturing apparatus. An example of a brush used for this purpose is shown in U.S. Pat. No. 3,795,931 to Martino. The brush is typically driven by an ac induction motor.
In using brushes of the type described in the above-cited patent, it is important that the brush be the proper distance from the printed circuit board (or other flat article). If the brush impinges upon or engages the board with too much force, the bristles of the brush may damage the board. Since the material to be scrubbed and removed from the board is usually very thin, sometimes of the order of thousandths of an inch, it is therefore easy to scrape or score the board by application of too much brush pressure. On the other hand, if the brush is not so located to impinge upon the board sufficiently, the board will not be adequately cleaned. Therefore, the brush must be maintained at a certain optimum location relative to the board.
In the prior art, it has also been necessary to brush the surface of a board through which holes have been drilled, in order to eliminate the burrs left after drilling. The same considerations which apply to scrubbing apply also to deburring.
It is understood that the terms "brushes," as used herein, refers, inter alia, to scrubbing or deburring brushes, and the like, and wiping and brushing means of various constructions, and not to "brushes" as used in the dc motor art. It is further understood that the term "brushing" is used to include, inter alia, such mechanical operations as scrubbing and deburring, as described above.
In the prior art, as represented by the above-cited patent, the proper setting of the distance of the brush relative to the board was determined by monitoring the current through the brushing drive motor. Elementary electrodynamics teaches that the torque generated by the motor is directly proportional to the current through the windings (torque being proportional to magnetic moment, which is itself proportional to the current through the coil). Thus, in the past, the current was taken as an indication of the loading on the motor. If the motor showed signs of overloading, the motor shaft would be moved farther away from the board so that the brushes would be moved to impinge less upon the board, and vice versa. However, it has been found that the use of the motor current as an indication of motor torque is not satisfactory. In an ac induction motor, there are both resistive and reactive effects, and it is well known that only the resistive component accounts for actual power consumption.
The total current supplied to an induction motor can be divided into resistive and reactive components. In a totally resistive current demand, all current is converted into energy, whereas in a reactive current demand, all energy supplied to the system is returned at some time later to the generator. Therefore, with a purely reactive load, no energy is consumed. Thus, in an electric induction motor, the resistive component is totally responsible for energy delivered to the shaft.
The present invention provides a brushing means for flat articles, such as printed circuit boards, in combination with electronic circuitry which more accurately measures the motor torque, so that the proper impingement of the brushes upon the printed circuit board (or other flat article) can be more reliably determined. The invention also discloses a servomechanism which enables the spacing of the brushes to be continuously and automatically maintained.